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STEP: Satellite Test of the Equivalence Principle
(Inertial Mass = Gravitational Mass)

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Galileo famously pronounced that all material objects fall at the same rate in a given gravitational field provided there is no air drag. Galileo's experiments were accurate to about 1%. Newton elaborated this as the Equivalence Principle, and improved the experimental accuracy to 0.1% using pendula made of different materials. Modern experiments using a torsion pendulum with the sun as the source of the gravitational field have found no violation of Equivalence at 1 part in 1013.

According to several theories this Equivalence may breakdown if still more accurate measurements can be made. Examples of theories are:

  • Effects inherent in a quantum theory of gravity when such a theory has been constructed. The present theories of General Relativity and Quantum Electrodynamics are not compatible - at least one will have to be modifed.
  • New short range forces predicted by some theories.
  • The Theory proposed by Damour et al.

Worden and Everitt have proposed to carry out an improved version of Galileo's Leaning Tower of Pisa experiment, by placing masses of different composition inside a satellite orbiting the Earth to obtain a long duration of free fall. Very high positional sensitivity can be achieved by cooling the experiment to 1.8 K and using SQUIDs to detect in current changes in magnetic pickup coils. The test masses of different materials and the position measuring equipment are placed inside the satellite and micro Newton thrusters are used to cancel the small accelerations of the satellite due to air drag, radiation pressure and the solar wind which could couple to masses. STEP aims to measure Equivalence at the level of 1 part in 1018.

The picture shows a concept for the 'drag-free' satellite which was developed in 1999 by RAL and ZARM. This was elaborated in an ESA Phase A study of the satellite carried out by Astrium. NASA has selected STEP for a SMEX phase A study due to commence on October 2001.

Last updated: by M.C.W.Sandford